Dispersion

- first full release.

- added config option to supress YAML package warnings.

- fixed incorrect name of the setup script in the documentation.

- added angstrom as a valid unit for wavelength spectrums.

- added support for loading .nk files.

- .txt and .csv files can now have colons in their metadata.

- updated PyYAML dependency to v5.4

changelog:

- order of elements no longer reversed when inverting spectrum

- Bruggeman effective medium model should provide physical solutions for
non-absorbing media.